The problem of determining the ageing of a battery has many applications, particularly in the context of batteries installed on board satellites. In such cases, knowing the state of the battery is crucial when it comes to determining the end of the satellite lifetime. In the absence of precise knowledge about the state of ageing of the battery, the satellite is prematurely deactivated, thus generating a significant operating loss.
The behavior of a battery as a function of its ageing is illustrated by FIG. 1, which shows the depth of discharge of the battery (separation between the maximum charge and the minimum charge of the battery) on the abscissa, and the voltage across the battery terminals on the ordinate. As can be seen in FIG. 1, for a battery at the beginning of its life (curve 1), the available discharge depth is maximal, and the curve of voltage supplied by the battery is substantially flat until the maximum discharge depth is almost reached, when there is a rapid drop in voltage. For a battery at the end of its life (curve 2), the situation is identical, with a plateau at a slightly lower battery voltage in normal operation, and a rapid drop in voltage in the vicinity of the discharge depth.
Curve 3 (in dotted lines in FIG. 1), corresponding to a battery of any age, is found, on this graph, between the two preceding curves. The simple measurement of the voltage observed on the plateau at the start of the battery use (left-hand part of the curve) does not suffice to accurately characterize the ageing of the battery. This thus results in a great deal of uncertainty regarding the time of the rapid drop in voltage of the battery.
The usual methods for determining the ageing of a battery comprise the measurement of its internal resistance and of the loss of capacity of the battery (drop in the discharge depth). The capacity measurement is based on a complete discharge of the battery at constant current.
These parameters of capacity and internal resistance are therefore not accessible for embedded equipment, on board a satellite for example. Moreover, a complete discharge of battery on board a satellite would lead to the loss of the latter, and this solution is therefore not envisionable.
Methods for diagnosing the ageing of a battery suitable for embedded equipment are moreover known.
Patent application US 2012/0019253 A1 “Method for determining an aging condition of a battery cell by means of impedance spectroscopy” describes a diagnostic method based on the comparison between a low-frequency impedance of the real battery, and a threshold impedance, characteristic of a battery at the end of its life.
The low-frequency impedance measurement of the battery is here performed by measuring the voltage of the battery, when the latter undergoes the injection of a current including sinusoidal variations of variable frequency (impedance spectroscopy method).
The method is applicable to various types of battery: Li-Ion, Li-Polymer, NiCd, NiMH, etc.
Similarly, the publication “Impedance Noise Identification for State-of-Health Prognostics” (INL/CON-08-14101, 43rd Power Sources Conference, July 2008) describes an ageing diagnosis method using an impedance measurement of the battery, here based on the injection of a random current (white noise and pink noise) into the battery. The method then uses FFT (Fast Fourier Transform) processing to determine the impedance of the battery. Here again, this measured impedance is compared to reference values.
In this field a document EP 1 892 536 A1 is known relating to the analysis of the ageing of a lead-type battery, such as used in a motor vehicle in particular. This document describes a system for computing an impedance spectrum using a measurement performed during the ignition pulses (high-frequency pulses that are reliable and required to be reproducible), and if necessary completed by test pulses using controllable charging dedicated to this effect. This document mentions the limits of a model using only one ignition pulse as data source, and introduces controllable charging intended for measurements with a test pulse. These test charges can be part of the normal electrical equipment of the vehicle, but they are driven to generate pulses at the moment of the ageing measurement. The measurement therefore takes place either at the time of the ignition pulse, or during test pulses.
A patent document U.S. Pat. No. 4,678,998 is also known, also relating to the accurate measurement of ageing of a lead-acid battery of a motor vehicle. This document mentions that the various loads connected to the battery will produce frequency components with different voltages and currents on the battery. This document describes a device wherein the voltage and the voltage across the terminals of a motor vehicle battery are constantly measured, and impedance measurements are computed at the various frequencies associated with the loads connected to the battery. Finally, the system compares its measurement results to pre-stored curves for each of the frequencies associated with the operation of the loads.
Another document EP 2 375 487 is known relating to a method of identification of a model representing the ageing of a battery, using a sequence (M-sequence) injected into the battery. This is clearly apparent in the text, particularly § 10. D3 describes a step of selecting a model of evolution of the impedance, and mentions an ARX model as an example.